The present invention relates to the production of free-flowing spheres of solid particulate materials using partially neutralised carboxylic acids.
In producing and processing finely divided solid materials in dry form, it is desirable that the particles be substantially uniform in shape and size and that they be free-flowing. Various finely divided materials are required to be substantially dispersed in polymeric or liquid media. In such dispersions, the term "primary particles" refers to the individual crystals and tightly held aggregates thereof "Agglomerates" are larger associations of the primary particles. Ideally, a dispersion consists mainly of primary particles and a minimum of loosely held aggregates consisting of co-adhering primary particles. In contrast, when finely divided materials are processed in dry solid form, the particles can be larger, but are preferably uniform in shape and size so that they are free-flowing.
Powders consisting of such finely divided materials generally exhibit poor flow properties and readily generate undesirable levels of dust. Powders which are coated with hydrophobic agents which assist pigment wetting and dispersion in polymer systems can exacerbate problems such as dustiness. Poor flow characteristics result in difficulties achieving accurate flow metering. Excessive dustiness can cause problems with respect to industrial hygiene. Free-flowing aggregates overcome these problems. A weakness of current methods of producing free-flowing powders is that the aggregates are difficult to de-aggregate to primary pigment particles, which limits their applications. A further weakness of some methods used to produce free-flowing powders is that significant quantities of binders, up to 20% by weight with respect to the particulate material, are required.
Some free-flowing plastic grade pigments or fillers are commercially available. Due to the role of pigments such as titanium dioxide as opacifying agents, the primary pigment particles must be sub-micron in size. Free-flowing plastic grade pigments or fillers are available such as Tioxide's TR36 or Bayer's R-FK21; however, both of these grades suffer from a difficulty to deaggregate and disperse, which limits their usefulness. The present invention substantially overcomes this weakness.
U.S. Pat. No. 4,375,989 discloses titanium dioxide pigments whose dispersibility is improved by coating with an organic coating and also an inorganic coating. Organic substances used for coating include large-molecule fatty acids and their salts. Suitable inorganic coatings are oxides and hydroxides of aluminium, zinc, titanium, zirconium and magnesium.
U.S. Pat. No. 4,186,028 discloses aqueous fluid suspensions of fillers or pigments containing dispersion aids which can include phosphonocarboxylic acids and/or salts thereof.
U.S. Pat. No. 4,563,221 discloses pigments comprising particulate titanium dioxide having an organic coating of isostearic acid, dodecyl benzene sulphonic acid and a cationic emulsifying agent of a fatty alkyl amine. The pigment particles are described as free of inorganic coatings, and after treatment do not require milling in a fluid energy mill.
U.S. Pat. No. 1,722,174 proposed the use of alkali metal and ammonium salts of fatty acids for rendering lithopone (a mixture of zinc sulfide and barium sulfate) organophilic.
U.S. Pat. No. 3,042,539 discloses the use of alkali metal and ammonium salts of fatty acids for treating zinc oxide to produce organophilic pigments of very fine particle size.
U.S. Pat. No. 4,255,375 discloses the treatment of aqueous dispersions of organic pigments with at least one organic acid (such as octanoic) which is liquid at temperatures below 100.degree. C., or a salt thereof, at a pH value at which the acid is water insoluble, maintaining the temperature above the melting point of the acid until the pigment has transferred into the organic phase, and then adding base to increase pH until the acid becomes water soluble. The amount of organic acid added to the pigment is greater than 10 weight percent of the resulting composition, based upon the disclosure of acid and salt as "from 0.1 to 4 parts . . . by weight per part of pigment." The pigment is recovered as spherical granules from 0.1 to 3mm in diameter.
U.S. Pat. No. 3,506,466 discloses titanium dioxide pigments, with or without inorganic coatings, which are treated with salts of water-soluble alkanolamines and oxycarboxylic acids and then milled in a fluid energy mill.
Hungarian Patent No 148,370 discloses organophilic oxide pigments prepared by adding an aqueous soap solution of an alkali metal or ammonium salt of a fatty acid (such as ammonium stearate) to an aqueous slurry of an oxide, such as titanium dioxide, then adding an acid such as HCl to adjust the pH to about 5.
U.S. Pat. No. 4,224,080 discloses inorganic oxide pigments (such as titanium dioxide) coated with alumina and treated with water soluble reaction products of excess di- or polybasic organic hydroxy acids with di- or polybasic alcohols.
British Patent No 909,220 discloses dry titanium dioxide pigments which are treated with water-soluble salts of organic (eg carboxylic) acids with tertiary amines. The treated pigments are said to disperse more easily in a vehicle and to exhibit a greater tendency to dry flow.
U.S. Pat. No. 4,786,369 discloses abrasive soap powders which can comprise titanium oxide particles coated with soaps formed by the reactions of organic acids with alkaline hydroxides or amines.
U.S. Pat. No. 4,923,518 discloses chemically inert pigmentary zinc oxide compositions prepared by wet treatment of chemically reactive zinc oxide base pigments which include the application of chemically inert organic or inorganic coatings. Such coatings can include water insoluble metallic soaps of a saturated or unsaturated monocarboxylic acid and/or various hydrous metal oxides. The pigment particles can be spherical or acicular in shape.
U.S. Pat. No. 4,277,288 discloses a process for producing a substantially dry, low dusting, free-flowing granular pigment composition by contacting a fluidised bed of pigment and a granulating assistant.
U.S. Pat. No. 5,215,583 discloses formation of granules from a suspension of one or more pigments wherein the suspension also contains 0.05 to 5% of a soluble salt, selected from alkali/alkaline earth metal chlorides, sulfates and phosphates.
U.S. Pat. No. 5,215,584 discloses a method for producing inorganic granules from a suspension of one or more inorganic pigmenting agents and a hydrolysed or poorly soluble compound of one or more ion types present per se as an essential constituent in one or more pigments.
U.S. Pat. No. 5,108,508 discloses a method for producing microgranules which are free-flowing and non-dust forming comprising providing an aqueous suspension of pigment which optionally contains from 0.1 to 0.9% by weight of binder and optionally contains from 0.1 to 2.0% by weight of silicone oil and spraying said suspension into a spray tower through a hollow cone nozzle. The binder is a sodium or ammonium polyacrylate, and the granules have a toroidal shape.
U.S. Pat. No. 5,634,970 discloses a granulating process comprising pretreating inorganic pigments by the addition of oils as binders and subjecting the pretreated pigment to a compacting stage to form flakes which are then broken down by rough grinding.
Many techniques have been developed for producing and processing such finely-divided materials, but in most cases they still lack uniformity as to size and, particularly shape, and hence are not as free-flowing as desired. Clearly there is room for improvement in this field.
Accordingly, an object of the present invention is to produce finely divided particulate materials such as pigments in which the particles are agglomerates substantially uniform in size and shape. Another object of the invention is to produce particles which are substantially free-flowing in bulk. A further object of the invention is to produce agglomerates of such finely divided materials which are substantially spherical in form. A further object of the invention is to produce free-flowing agglomerates which are sufficiently robust to survive manual handling whilst still being sufficiently soft to readily de-aggregate and disperse within the media into which the powder is to be incorporated.